1. Field of the Invention
The present invention relates to a vibration generator which is built into a mobile phone etc., more particularly relates to the structure of a vibration generator by which a high vibrating force is obtained.
2. Description of the Related Art
As shown in FIG. 17, the vibration linear actuator 51 disclosed in Japanese Patent Publication (A) No. 2003-154314 is provided with a ring-shaped external yoke 54 as a weight having a cylindrical permanent magnet 55 at its inner circumference side, a first plate spring 57 fastened to a first end face 54a of this external yoke 54 by elastically deforming an outer circumference side hanging part 57a, a second plate spring 56 the same as this first plate spring 57 and fastened to a second end face 54b of this external yoke 54 by elastically deforming an outer circumference side hanging part 56a, and a stator supporting the first and second plate springs 57 and 56 and having a coil 52 which generates a reciprocating vibration magnetic field at an inner circumference side of the permanent magnet 55, wherein this stator is provided with a plastic base 59 at a bottom surface of which a power feed land 61 is arranged and from which a shaft 58 stands and, with an internal yoke 53 on this base 59 having the shaft 58 at its center and forming also a coil bobbin of the coil 52. An inner circumference side 57b of the first plate spring 57 mates with a first projection 53a of the internal yoke 53, while an inner circumference side 56b of the second plate spring 56 mates with a second projection 53b of the internal yoke 53 and is sandwiched between the base 59 and internal yoke 53.
In the state with no power fed to the coil 52, the external yoke 54 serving as the weight remains at a standstill at an illustrated neutral position in the axial direction where the elastic recovery force outward at the first end face 54a side due to the first plate spring 57 and the elastic recovery force outward at the second end face 54b side due to the second plate spring 56 are balanced, but due to the alternating current flowing through the coil 52, an S-pole and an N-pole are alternately generated at the two ends of the internal yoke 53 in the axial direction, so the ring-shaped permanent magnet 55 magnetized in the radial direction, solely receives a magnetic attraction/repulsion action in the axial direction due to the strong magnetic pole at the inner circumferential surface side close to the internal yoke 53 rather than the magnetic pole at the outer circumferential surface side, so the first plate spring 57 and the second plate spring 56 alternately are restored to the free state of their planar shapes in a repeated operation and the external yoke 54 engages in reciprocating linear motion in the thrust direction resulting in reciprocating vibration.
As related art, there is Japanese Patent Publication A No. 2003-154314 (FIG. 1).
However, in the above vibration linear actuator 51, due to the alternating current flowing through the coil 52, the two ends of the internal yoke 53 across the axial direction alternately switch between the S-pole and the N-pole, but the outer circumferential surface of the external yoke 54 has the opposite magnetic pole to the magnetic pole of the inner circumferential surface of the permanent magnet 55. The magnetic pole of the outer circumferential surface of the external yoke 54 has the effect of suppressing linear reciprocating vibration of the permanent magnet 55, so it is necessary to lengthen the diametrical direction distance from the inner circumferential surface of the permanent magnet 55 to the outer circumferential surface of the external yoke 54 so as to weaken that effect. The longer the outer diameter of the external yoke 54 is set, the more this runs counter to the reduction of size of the vibration linear actuator of course, the magnetic circuit of the permanent magnet 55 becomes an open loop, and the vibration force becomes weaker. Obtaining the required vibration force leads to a larger size due to the increased number of turns of the coil 52 or an increase in the power consumption due to the increase in the alternating current.